This invention relates generally to sol-gel processes for producing glass rods suitable for use in producing optical fibers and, more particularly, to such processes for producing glass rods that are doped with germanium dioxide, making the rods suitable for use in producing optical fibers of high numerical aperture.
One common family of processes for producing rods of this particular kind involves the vapor deposition of fine particles of silicon dioxide (SiO.sub.2), or silica, and germanium dioxide (GeO.sub.2), or germania. In one such process, called the outside vapor deposition (OVD) process, fine particles of silica and germania, often called soot particles, are deposited layer by layer on a horizontal, rotating mandrel. The deposition is performed at sufficiently high temperatures to partially sinter the particles and form a porous cylinder. A core of germania and silica is deposited first, followed by a pure silica cladding. At the conclusion of deposition, the mandrel is removed and the tube is sintered, or densified, at 1500.degree. to 1600.degree. C. to a dense glass rod, or preform.
A second vapor deposition process of this kind, called the vertical axial deposition (VAD) process, also forms a cylindrical body using soot particles, but deposition occurs from one end, or axially. The core and cladding are deposited simultaneously, using two torches. When complete, the cylindrical body is sintered to a dense glass rod, under conditions similar to those used in OVD process.
In a third vapor deposition process, called the modified chemical vapor deposition (MCVD) process, high-purity purity gas mixtures are injected into a rotating, high-quality, pure silica tube, which is mounted in a glass working lathe and heated by a traversing oxyhydrogen torch. A gas phase reaction occurs in the hot zone created by the torch, to produce particles that deposit downstream of the hot zone. The heat from the moving torch sinters this deposit to form dense glass layers. A number of layers of germania and silica soot particles are deposited in this fashion. The cladding layer consists of pure silica.
In all of these vapor deposition processes, the core glass is composed of germania-doped silica, because germania increases the refractive index of the glass. The refractive index increases progressively with germania concentration. The glass used as cladding around the core is usually pure silica, which has a lower refractive index than germania-doped silica. This refractive index difference results in the waveguiding properties of optical fibers. As the difference of refractive indices between clad and core glasses increases, an increasingly higher numerical aperture of the resulting fiber is provided. Many applications in the fiber optics industry require optical fibers having numerical apertures greater than 0.2. Though commercial vapor deposition technologies have produced optical fibers with numerical apertures greater than 0.2, a technology to produce such fibers using a sol-gel process is not yet readily available.
Research has recently been conducted into the use of sol-gel processes for fabricating optical fibers. In such processes, a desired solution, or sol, of glass-forming compounds, solvents and catalysts is poured into a mold and allowed to react. Following hydrolysis and condensation reactions, the sol forms a porous matrix of solids, or gel. With additional time, the gel shrinks in size by expelling fluids from its pores. The wet gel is then dried in a controlled environment, to remove fluid from its pores, and it is then densified into a solid monolithic glass rod.
When optical fibers of high numerical aperture are to be produced, the core glass rod is formed by adding a germania-containing precursor in the sol, in a desired percentage, and the clad tube is formed from pure silica. The germania-containing core glass rod can then be sleeved with the cladding tube, and the fiber can be pulled by heating the ensemble to the glass softening temperature under vacuum. The vacuum forces the sleeve tube to collapse on the core rod at the softening temperature and the fiber draw is then initiated.
Advantages of the sol-gel process include chemical purity and homogeneity, flexibility in the selection of compositions, and processing at relatively low temperatures. By contrast, the vapor deposition processes described earlier are relatively expensive due to a relatively low soot collection efficiency and due to a resultant high investment cost in processing and pollution control equipment. Sol-gel processes have an inherent advantage over the soot deposition processes because glass is formed merely by the mixing of chemicals.
One major problem encountered in producing germania-doped silica glass using a sol-gel process, however, has been severe bubbling encountered when the glass rods are heated to a temperature exceeding 1800.degree. C., for fiber draw. Glasses produced from the vapor deposition processes have not exhibited this behavior. This bubbling tendency becomes progressively worse as the percentage of germania in the glass is increased. However, to manufacture optical fibers having a high numerical aperture, i.e., greater than about 0.2, it is necessary for the core glass to contain germania in at least about 10 mole percent.
Another major problem encountered in producing germania-doped silica glass using a sol-gel process has been premature precipitation of germania powder from the sol. This is believed to be due to the faster reaction rate with water of the germania precursor, tetraethyl orthogermanate, than the silica precursor, tetraethyl orthosilicate. Because of the premature precipitation, a highly non-uniform composition is provided.
Although there have been efforts to eliminate the bubbling and premature precipitation problems, such efforts have not generally succeeded in producing glass with germania concentrations of more than 10 mole percent. It should therefore be appreciated that there is a need for an improved sol-gel process for forming germania-doped silica glass rods that avoids the problem of premature precipitation of germania powder and that provides glass rods that will not bubble when exposed to temperatures exceeding 1800.degree. C., so that optical fibers having numerical apertures greater than about 0.2 can be drawn from such glass rods. The present invention fulfills this need.